It is a bug for fscrypt_put_master_key_activeref() to see a NULL
keyring. But it used to be possible due to the bug, now fixed, where
fscrypt_destroy_keyring() was called before security_sb_delete(). To be
consistent with how fscrypt_destroy_keyring() uses WARN_ON for the same
issue, WARN and leak the fscrypt_master_key if the keyring is NULL
instead of dereferencing the NULL pointer.
This is a robustness improvement, not a fix.
Link: https://lore.kernel.org/r/20230313221231.272498-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
4bcf6f827a